Method and apparatus for registering a communication terminal with a satellite communication system

ABSTRACT

A satellite communications system provides a communications service to a mobile terminal ( 2 ) on which different communications applications ( 4   a  to  4   d ) may be run. Calls are set up between any of the applications ( 4   a  to  4   d ) via a satellite ( 12 ) to a network management center ( 18 ) which provides different service adaptors ( 20   a  to  20   d ) which adapt the calls to different types of service provided over terrestrial networks ( 22 ), such as telephony, facsimile, internet or ATM services. The bandwidth allocated to each call over the satellite link may be varied during the call according to demand either from the relevant application ( 4 ) or from the network management center ( 18 ). Multiple calls may be connected concurrently to or from different applications running on the mobile terminal. A maximum bandwidth is allocated to each call. Efficient use is thereby made of the limited bandwidth available over the satellite ( 12 ), according to the instantaneous bandwidth requirements of different applications ( 4 ). For real-time calls requiring multiple slots per frame in a TDMA channel, the slots are mutually spaced apart to reduce delay.

The present invention is a divisional of and claims priority from, U.S.application Ser. No. 09/319,460 filed Aug. 9, 1999, now abandoned whichin turn is a 371 of PCT/GB97/03344 filed Dec. 4, 1997 and claimspriority from GB 9625475.0 filed Dec. 6, 1996.

FIELD OF INVENTION

The present invention relates to a communication method and apparatusand particularly, but not exclusively, to a communication method forallocating bandwidth to a call with a mobile terminal.

BACKGROUND ART

In some satellite and terrestrial mobile communication systems,terminals are available which provide both voice and data communication.For example, some GSM mobile telephones are connectable to dataterminals, such as portable computers. During a data call using suchmobile terminals. TDMA slots are assigned to the data call in a similarway as to a voice call, so that a constant bandwidth connection isestablished both in the forward and return direction. Mobile satellitesystems, such as the Inmarsat-B™, InmarsatC™ and Inmarsat-M™ satellitecommunication systems, allow fixed-bandwidth data communications.

Such systems are suitable for the exchange of some types of data, butare primarily designed for voice communications, which require aconstant symmetrical data rate. Such systems are not optimized for bothdata and voice communication.

There is an increasing demand for data communications which requireintermittent bursts of data to be sent or received at a high data rate,while requiring only low data rate communication at other times. Forexample, it is desirable when using a Web browser for requested pages tobe downloaded as quickly as possible to the user terminal, but little orno bandwidth is required in the forward direction while the user readsthe downloaded page. Such usage is also very asymmetrical, since theuser only needs to send requests for new pages or small amounts of datain the return direction. The use of such applications over the GSMsystem is unsatisfactory, since for some of the time during the datacall the allocated bandwidth is not used, but while large amounts ofdata are being downloaded, the allocated bandwidth is insufficient.

Furthermore, it would be desirable to allow multiple calls to be handledconcurrently by the same mobile terminal. For example, during atelephone call, a mobile user may wish to refer to data from an onlinedatabase, or receive an incoming facsimile.

A variable data rate satellite communication system is described in U.S.Pat. No. 4,256,925. In this system, a satellite is used forcommunication between a plurality of ground stations. Each groundstation requests a proportion of the total channel capacity inaccordance with the traffic load of voice and data call to that groundstation. A reference station in the network allocates the channelcapacity among the stations.

An ATM (asynchronous transfer mode) satellite communication system isdescribed in U.S. Pat. No. 5,363,374. Each earth station requestssporadic connection to the communication system and a central managementstation determines whether to accept or refuse the connection accordingto the available bandwidth.

The document GB 2 270 815 A discloses a cellular mobile radio systemwhich provides a packet reservation multiple access protocol forimplementing a variable bit rate service. The mobile terminals contendfor access in the same time slots in which they will transmitinformation. In order to support variable bit rates, the terminals havethe ability to reserve multiple slots in any given frame. Both voice anddata calls may be supported in the same frame, with a larger burst sizebeing used for data traffic for greater efficiency, and a small burstsize being used for voice traffic to reduce the delay incurred inwaiting for enough information from a voice code.

The document EP 0 713 347 A discloses a system for transmission of STMand ATM traffic on a broadband communications network, such as afiber/coax network or a wireless network in which mobile stations dependon a base station for feedback. ATM calls can be constant bit rate,delay sensitive variable bit rate, delay tolerant variable bit rate orcontention based. Delay sensitive variable bit rate calls are allocatedtime slots in accordance with a statistically weighted incrementalbandwidth determination that takes into account existing variable bitrate traffic and the statistical characteristics of the new callrequest. ATM traffic is assigned a minimum guaranteed bandwidth and anyspare bandwidth is assigned among existing ATM calls or used to admit anew ATM call.

STATEMENT OF INVENTION

According to one aspect of the present invention, there is provided amobile communication system in which the bandwidth available for anindividual call between a mobile communications terminal and a basestation is varied according to the demand for bandwidth during that callwithout exceeding a predetermined maximum level determined for thatcall. Advantageously, the bandwidth may be increased when a largequantity of data is to be sent, but is reduced at other times and theadditional capacity made available to other users.

The maximum level may be determined according to the type of call inorder to support a peak bandwidth suitable for that type of call.

According to another aspect of the present invention there is provided amethod of assigning time slots to individual calls in a TDMA mobilecommunication system, in which multiple slots within a frame whichrelate to a real-time call are spaced apart from each other in theframe. Advantageously, this slot allocation scheme reduces the delayincurred by the TDMA frame structure.

According to another aspect of the present invention, there is provideda method of registering a mobile terminal in a satellite communicationssystem which has both spot beams and a global beam, in which theterminal first attempts to communicate in the last spot beam channelused, and only re-registers in the global beam to receive a new spotbeam channel allocation if the last spot beam channel cannot bereceived.

The present invention extends to apparatus for carrying out any of theabove methods.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a satellite communication systemaccording to an embodiment of the present invention;

FIG. 2 is a diagram of a protocol layer structure employed by theembodiment of FIG. 1;

FIG. 3 is a diagram of structure of a TDMA frame according to the airinterface protocol of the embodiment of FIG. 1;

FIG. 4 is a diagram of the states of the mobile terminal in theembodiment of FIG. 1, showing the possible transitions between them;

FIG. 5 is a flow diagram of the operation of the mobile terminalbeginning from the unlocated state of FIG. 4;

FIG. 6 is a flow diagram of the operation of the mobile terminalbeginning from the idle state of FIG. 4;

FIG. 7 is a flow diagram of the operation of the mobile terminalbeginning from the signalling state of FIG. 4; and

FIG. 8 is a flow diagram of the operation of the mobile terminalbeginning from the active state of FIG. 4.

MODES OF CARRYING OUT THE INVENTION

System Overview

FIG. 1 shows schematically a mobile terminal connected via a satellite12 to a network management center 18, which allocates bandwidth to themobile terminal and connects the mobile terminal to a terrestrialnetwork 22. In this embodiment, the mobile terminal 2 comprises aportable computer on which a number of different communicationsapplications 4 a, 4 b, 4 c, 4 d may be run. For example, theapplications may be a voice telephony application, an internetapplication, a facsimile application and an ATM application. Each ofthese applications use standard application programming interfaces (API)such as Winsock for internet access, TAPI for telephony applications andCAPI for ISDN applications. The interfaces to such applications areshown schematically by the reference 12 in FIG. 1. Driver software 6running on the mobile terminal 2 converts API protocols to proprietaryprotocols designed for the satellite communication system. The mobileterminal 2 provides a physical interface 14 to an interface card 8, suchas a PC (formerly PCMCIA) card. The interface card 8 includes a radiofrequency modulator/demodulator connected to an antenna 10. The radiofrequency modulator/demodulator is able to receive on a first frequencychannel and to transmit simultaneously on a second frequency channel.

The antenna 10 is located within the coverage region of a spot beam Bgenerated by the satellite 12, which may for example be a geostationarysatellite having multibeam receive/transmit antennas for receiving andtransmitting signals in each of a plurality of spot beams B. Each spotbeam B carries a plurality of frequency channels both in the forward andreturn directions. The satellite also receives and transmits in a globalbeam G which has a coverage area extending substantially or completelyover the coverage areas of the spot beams B. The global beam G carriesat least one forward and one return frequency channel.

The RF signals transmitted between the antenna 10 and satellite 12comply with an air interface protocol 13, which will be more fullydescribed hereafter. The satellite 12 acts as a repeater and convertschannels from multiple spot beams B into channels in a feeder beam F andvice versa. The feeder beam F provides a link between the satellite 12and an earth station 16 via an earth station antenna 14. The airinterface protocol over the feeder beam F is referenced as I3F in FIG.1.

The network management center 18 is connected to the earth station 16and includes a number of different service adaptors 20 a, 20 b, 20 c, 20d providing an interface to terrestrial networks 22, such as PSTN, ATMnetworks or ISDN. For example, the service adaptors 20 may comprise atelephony adaptor 20 a including a codec for converting voice signals ona PSTN to data at the network management center 18 and vice versa. Afacsimile service adaptor 20 b may implement facsimile protocols, suchas defined in ITU Recommendations T.30 and T.4 and include a modem forcommunication over a PSTN. An internet service adaptor 20 c implementsTCP/IP and an ATM service adaptor 20 d implements ATM protocols. Thesestandard protocols and interfaces are designated collectively by 11 inFIG. 1.

The mobile terminal 2 allows multiple different types of communicationto be set up over the satellite communication system, such as telephony,internet, fax and ATM. These applications may be run concurrently. Thebandwidth allocated to each application may be varied independently inthe forward and return directions during a call, as will be describedbelow.

Air Interface Protocols

The implementation of the air interface protocols 13 and 13F, asperformed by the driver software 6 of mobile terminal 2 and by thenetwork management center 18, will now be described with reference toFIGS. 2 and 3. The protocol structure is described in terms of “layers”which interwork with each other as shown in FIG. 2.

The top layer comprises a slot management layer 28 which receives data Dfrom and sends data D to the applications 4 or the service adaptors 20.The data is formatted in slots S, each comprising a cell, as shown inFIG. 3. Each cell C comprises a header H and data D both of fixedlength. The slot management layer 28 formats data into and out of slotscontaining such cells, and exchanges the slots with a TDMA layer 26which controls the timing of transmission and reception of the slots Swithin TDMA frames FR, which are sent to or received from a physicallayer 24.

The physical layer 24 corresponds to the interface 14, the interfacecard 8 and the antenna 10, providing a physical interface between thedriver software 6 and the air interface 13, or corresponds to the earthstation antenna 14 and earth station 16, which likewise provide aphysical interface between the network management center 18 and thesatellite 12. In both cases, the physical layer 24 converts the framesFR to radio frequency signals RF and vice versa.

The slots S contain, in addition to the traffic data D, signallinginformation which is used to set up calls and to vary the assignment ofbandwidth during a call. The creation and reception of these signals isperformed by a session management protocol layer 30, which interworkswith the slot management layer 28 and the TDMA layer 26 to receive ortransmit signalling information within the cells C, in the header Hand/or as data D.

As shown in FIG. 3, each TDMA frame is transmitted or received in aformat comprising 18 slots S₁ . . . S₁₈, each comprising a cell C, witha guard band G separating each slot. Each slot S also containssynchronization and control information which is used to acquire thetiming of the slots, and will not be discussed further.

Each slot S may be assigned to any mobile terminal 2 with which a callhas been set up, under the control of the network management center 18.

Alternatively, more than one cell may be transmitted in each slot, witheach cell being assignable to a different call to or from the samemobile terminal or even to a different mobile terminal.

Call Management

The different states through which the mobile terminal 2 passes duringoperation are shown in FIG. 4. In order to communicate with the mobileterminal 2, the network management center 18 determines in which spotbeam B the mobile terminal 2 is located. If the mobile terminal 2 isbeing used for the first time, or has moved into a different spot beam Bsince it was last used, then the mobile terminal 2 is in an “unlocated”state. When the mobile terminal 2 has acquired a spot beam channel butis not handling any calls, it is in an “idle” state. When a first callis being set up with the mobile terminal 2, it enters a “signalling”state, unless the call set up fails in which case it returns to the“idle” state. When the first call has been set up, the mobile terminal 2enters an “active” state and remains in the active state until all callsare terminated, when the mobile terminal 2 enters the “idle” state onceagain. If contact is lost with the spot beam channel, the mobileterminal 2 returns to the unlocated state. Each of these states will nowbe described in detail.

Unlocated State

Transitions from the unlocated state will now be described withreference to FIG. 5.

When the mobile terminal 2 is activated, i.e., switched on or otherwiseenabled to communicate, the driver software 6 first tunes the interfacecard 8 to the frequency of the spot beam B last used for communication(step 34), if there has been any previous communication. If theinterface card 8 is able to receive in this spot beam B, the driversoftware 6 detects whether any of the applications 4 have requested thatan outgoing call should be set up from the mobile terminal 2 (step 36).If so, the mobile terminal 2 enters the signalling state; otherwise itenters the idle state.

If the interface card 8 is unable to receive signals at the frequency ofthe previously used spot beam B, then the interface card 8 is tuned tothe forward and return frequencies of the global beam G received by thesatellite 12 (step 38). The mobile terminal 2 then transmits a “log-on”message (step 40) in the global beam return channel. The log-on messageincludes identification information identifying both the mobile terminal2 and its current user, together with location information which issufficient for the network management center 18 to determine in whichspot beam B the mobile terminal 2 is located. This information may beentered by the user of the mobile terminal (i.e., by indicating in whatcountry the mobile terminal 2 is located), or may be derived bypositioning equipment in the mobile terminal 2, such as a GPS (GlobalPositioning System) receiver. Preferably, the positioning information issufficient to identify in which spot beam B the mobile terminal 2 islocated, but does not have sufficient accuracy to allow eavesdroppers tolocate the mobile terminal 2 precisely and thereby pose a security risk.If one of the applications 4 has requested that a call be set up, thelog-on message may also indicate that the mobile terminal 2 intends toestablish a call.

The mobile terminal 2 then awaits a response from the network managementcenter 18 in the forward channel of the global beam G (step 42). Theresponse from the network management center 18 includes identificationinformation so that the response may be correlated with the log-onmessage, spot beam channel identification information which identifiesthe frequency channel to be used by the mobile terminal 2 forcommunication in the spot beam B in which it is located, and timinginformation derived from the timing of the log-on message as received bythe earth station 16, to assist the mobile terminal 2 in synchronizingwith the timing of the frames FR. If the mobile terminal 2 indicated inthe log-on message that a call is to be setup, the response includes alabel which is used in a slot negotiation phase, as will be describedlater.

The interface card 8 is then tuned to the spot beam channel indicated bythe response (step 44). If no calls are currently required either fromor to the mobile terminal, the mobile terminal 2 enters the idle statewhile continuing to monitor the spot beam forward channel, otherwise itenters the signalling state (step 46).

Idle State

In the idle state, as shown in FIG. 6, the mobile terminal 2continuously detects whether it is able to receive frames correctly inthe designated spot beam channel (step 48). If it is no longer able todo so, the driver software 6 enters the unlocated state. Otherwise, themobile terminal 2 detects whether any of the applications 4 require anoutgoing call to be set up (step 50). If so, the mobile terminal 2transmits a request message (step 52) in a slot S reserved for suchsignalling in the return spot beam channel. The allocation of such slotsS is periodically indicated by the network management center 18 in theforward direction spot beam channel. Access to such slots is determinedby a slotted aloha access scheme, with the possibility of collision iftwo mobile terminals 2 attempt to transmit in the same slot, in whichcase the network management center 18 will not receive either requestmessage. The request message contains identification informationidentifying the mobile terminal 2.

In response to the request message, the network management center 18sends a “welcome” message, including a label which is used as atemporary identity code for the mobile terminal 2. If the mobileterminal 2 detects the welcome message (step 54) it enters thesignalling state; otherwise, the request is repeated (step 52) after apredetermined period. The period is increased after each unsuccessfulrequest (step 52) and includes a randomized component, to avoid repeatedconflict for the same return channel slot with the same other mobileterminals sending request messages. After a predetermined number ofunsuccessful requests, the terminal 2 returns to the idle state.

If a call originating from the terrestrial network 22 is to be connectedto the mobile terminal 2, the network management center 18 sendsidentifying information over the forward spot beam channel to the mobileterminal 2, to indicate that an incoming call is to be set up. If themobile terminal 2 detects such an incoming call (step 56) it enters thesignalling state; otherwise if there are no incoming or outgoing calls,the mobile terminal 2 stays in the idle state.

Signalling State

In the signalling state, as shown in FIG. 7, a setup protocol exchangetakes place (step 48), in which user authentication information is sentby the mobile terminal 2 to the network management center 18 and theaddresses of the call and calling parties are exchanged. A committed bitrate (CBR) and maximum bit rate (MBR) are established for each directionof the call. The committed bit rate is a bit rate which is guaranteedthroughout the call. The maximum bit rate is the maximum rate that canbe assigned to the new call at any stage during the call. Thesevariables are used by the network management center 18 during the callto determine the number of slots allocated to the mobile terminal 2. TheMBR and CBR in each direction may be determined according to the type ofthe call and/or according to a request by the mobile terminal 2 duringcall set-up. For example, if the call is an internet connection to beused for web access by the mobile terminal 2, a low CBR and MBR are setin the return direction while a low CBR and a high MBR are set in theforward direction, the level of the MBR being set according to a requestfrom the mobile terminal 2.

If setup fails for any reason (step 50), for example because therequested committed bit rate is not available in the spot beam B, or thenetwork management center 18 is unable to connect the call through theterrestrial network, the mobile terminal 2 returns to the idle state.

After the setup exchange, the mobile terminal 2 may retune (step 52) toa different channel within the spot beam B if the channel assignedduring setup is not the same as the channel being used for the call. Ifsetup is successful, the mobile terminal 2 enters the active state.

Active State

Once the mobile terminal 2 is in the active state, additional signallingmay take place between the mobile terminal 2 and the network managementcenter 18 so as to terminate a call, to exchange charging information orsystem information, to request more or less bandwidth for a call or toestablish another call. This information is preferably sent in one ormore of the slots already assigned to the mobile terminal 2.Alternatively, in the forward direction the network management center 18may send signalling information in a cell C of a slot S which is notcommitted to any mobile terminal 2, and indicate in the headerinformation of this cell that the cell contains signalling informationaddressed to the mobile terminal 2.

An example of a protocol exchange which may take place in the activestate of the mobile terminal 2 is shown in FIG. 8. The mobile terminal 2detects whether an incoming or outgoing call has been terminated (step58). If so, it detects (step 60) whether there are now no incoming oroutgoing calls. If so, the mobile terminal 2 enters the idle state,while remaining tuned to its current forward and return spot beamchannels; otherwise, it continues in the active state.

The mobile terminal 2 also detects whether any additional calls are tobe set up (step 62), either signalled by one of the applications 4 inthe case of an outgoing call or indicated in a forward direction cell Cfrom the network management center 18 in the case of an incoming call.The mobile terminal 2 determines, by means of a signalling exchange withthe network management center 18, whether the committed bit raterequired for the new call is available (step 64). If not, the driversoftware 6 determines whether a lower CBR than the requested CBR issuitable and available for the new call (step 66). If so, the new callis set up (step 68) by assigning additional slots to the new call withinthe same frequency channels, in addition to the slots which may alreadybe assigned to other calls connected to the mobile terminal 2.

If the lower committed bit rate is not available or acceptable, the callis terminated (step 70) and the mobile terminal 2 continues in theactive state, unless there are no other calls in progress in which casethe idle state is entered.

If the requested capacity is not available in the frequency channel towhich the mobile terminal is currently tuned, but is available inanother channel, the network management center 18 may signal to themobile terminal 2 that the option is available of retuning to anotherchannel and reassigning slots in that channel. If the mobile terminal 2accepts, it retunes to the new channel and receives a new slotassignment in that channel from the network management center 18.

While the mobile terminal 2 is in its active state, one of theapplications 4 may enter a state in which the driver software 6 detectsthat additional bandwidth is required (step 72). For example, theapplication may begin to output a large graphics or audio file. In thatcase, the mobile terminal 2 signals in one of the assigned slots of thereturn channel that additional slots are required (step 74) in thereturn direction. If the current bit rate assigned to that call is lessthan the maximum bit rate, and additional capacity is available in thefrequency channel to which the mobile terminal 2 is tuned, the networkmanagement center 18 assigns additional slots to the call in the returndirection and signals to the mobile terminal which additional slots maybe used in the return direction.

Slot Assignment

The system by which slots are assigned to the mobile terminal 2 in boththe forward and return direction will now be described in detail. Oncethe mobile terminal 2 has left the unlocated state, it is tuned toreceive one spot beam frequency channel in the forward direction and totransmit in another spot beam frequency channel in the return direction.Each mobile terminal 2 continuously receives in the forward frequencychannel, but transmits only in the slots in the return channel whichhave been assigned to that mobile terminal, and optionally in the slotswhich have been designated for sending request messages, as describedabove. The forward and return slots can contain both signalling and calltraffic.

As shown in FIG. 3, each cell comprises a header H and data D. Theheader comprises four bytes, formatted as shown below in Table 1.

TABLE 1 Bit 8 7 6 5 4 3 2 1 Octet Label PTI (bits 0,1) 1 PTI CLP ReturnAssignment or 2 (bit 2) Bandwidth Demand VCI 3 HEC 4

A description of each of the fields shown in Table 1 is given below.

Label

The label field contains the label of the mobile terminal which isintended to receive the cell or which transmitted the cell. As describedabove, the label is a temporary identity code which is assigned to eachmobile terminal 2 when it is in the active or signalling state. When themobile terminal 2 returns to the idle state, its label may be reassignedby the network management center 18 to another mobile terminal 2. Inthis way, the number of bits needed to address a mobile terminal isreduced. Since each frame FR of a frequency channel contains 18 slots,only 18 different labels are need to identify each of the differentmobile terminals tuned to that frequency channel. Six bits are assignedto the label field, allowing additional labels to be used for otherpurposes. For example, a further label may indicate that the cellcontains a broadcast message addressed to all the mobile terminalsreceiving the frequency channel, or for signalling purposes.

After a mobile terminal returns to the idle state, its label is not madeavailable for reassignment for a predetermined period, to avoid thepossibility of the same label being used for two different mobileterminals on the same frequency channel due to loss of call statesynchronization between the network management center 18 and the mobileterminal entering the idle state.

Return Assignment

In the forward direction, this field contains the label of the mobileterminal which is allowed to transmit in the corresponding slot of thereturn channel, for example the slot having the same order in the frameof the return channel.

Bandwidth Demand

This field is used in the return direction to indicate the bandwidthrequired by the transmitting mobile terminal 2 for all its calls in thereturn direction. One bit of the bandwidth demand field indicates thatadditional slots are required in the return direction for signallingpurposes, thus allowing signalling to take place without reducing thebandwidth available to currently active calls.

Virtual Channel Identifier (VCI)

This field identifies the individual call with which the cell C isassociated, thus allowing a single mobile terminal to support multipleconcurrent calls. The driver software 6 identifies the VCI of each cellC addressed to the mobile terminal and directs the contents of the cellto the corresponding application 4. Likewise, data from an activeapplication is assigned a VCI when formatted into a cell by the driversoftware 6.

Payload Type Identifier (PTI)

This field is present for ATM compatibility, and is passed transparentlyby the satellite communication system, to allow interworking withterrestrial ATM services through the terrestrial network 22.

Cell Loss Priority (CLP)

This field is also present for ATM compatibility, and is passedtransparently by the satellite system.

Header Error Control (HEC)

This field contains a check value calculated from the values from theother three bytes of the header, to allow corrupted headers to bedetected.

Bandwidth Allocation

The network management center 18 receives the bandwidth demand fieldsfrom each of the mobile terminals 2 transmitting in a single returnfrequency channel, and allocates return channel slots according to thesebandwidth demands, and the committed and maximum rate of each call. Theallocation of slots in the return channel is indicated by the returnassignment field.

The network management center 18 also determines which slots areaddressed to each of the mobile terminals in the forward direction,according to the required capacity in a forward direction for each ofthe calls. Thus, the capacity allocated in the forward direction may beselected independently from the capacity allocated in the returndirection, allowing asymmetric calls to be assigned only so muchcapacity as is needed in each direction.

The network management center 18 buffers data received from the network22 for transmission to the mobile terminal 2, and determines the numberof slots assigned to each call in the forward direction, and thereforethe current forward bit rate, so that the committed bit rate is providedbut the maximum bit rate is not exceeded. The current forward bit ratemay be determined according to the quantity of data buffered for thatcall at the network management center, so that a burst of data from thenetwork 22 will result in an increased forward bit rate. Optionally, ifcapacity on a forward channel is still available even after the maximumbit rate has been assigned to all calls, the maximum bit rate may beexceeded by assigning further slots to calls for which large amounts ofdata are buffered.

By appropriate selection of the committed and maximum bit rate for eachcall, various different types of call may be implemented. For example, avoice call may have a maximum bit rate equal to its committed bit rate,which is the bit rate required for the voice signal. Non real-time lowbit-rate applications, such as E-mail, may be assigned a low committedbit rate, but a high maximum bit rate so as to clear buffered e-mail atthe network management center 18 when there is any unused channelcapacity.

Billing Strategy

The network management center 18 may compile billing informationaccording to the committed and maximum bit rates assigned to each call.For example, the billing rate may be proportional to the committed bitrate, with a comparatively small surcharge proportional to the maximumbit rate. In this way, the total capacity of a frequency channel isshared between different mobile users according to their bandwidthrequirements, with lower priority non real-time users being charged lessfor access to the channel.

Real-time Calls

When determining the slot allocation for individual calls, the networkmanagement center 18 takes into account whether the call is a real-timecall (such as a voice call. For such calls, the delay in transmission toand from the mobile terminal 2 should be kept to a minimum. Therefore,where a real-time call occupies multiple slots in a frame, those slotsare spaced apart and distributed as evenly as possible throughout theframe so as to reduce the maximum delay encountered by any of the datain the real-time call. For example, Table 2 below shows the slotallocation where four different voice calls, V1 to V4, are establishedto four different mobile terminals 2, with each call V occupying fourslots S within the frame FR, while Table 3 shows, by way of comparison,an arrangement in which the slots assigned to each call V are groupedtogether.

TABLE 2 V₁ V₂ V₃ V₄ V₁ V₂ V₃ V₄ V₁ V₂ V₃ V₄ V₁ V₂ V₃ V₄

TABLE 3 V₁ V₁ V₁ V₁ V₂ V₂ V₂ V₂ V₃ V₃ V₃ V₃ V₄ V₄ V₄ V₄

In the case shown in Table 3, the data for each call may be buffered forup to one complete frame period. In contrast, the method in accordancewith this embodiment, as shown in Table 2, requires data to be bufferedfor only a small fraction of the frame period, thereby reducing bothdelay and the size of the buffer required. Real-time calls are thereforegiven priority when allocating slots, so as to achieve regular spacingof the relevant slots through the frame. Non real-time variablebandwidth calls are then allocated to the remaining slots available soas to assign the committed bit rate to each of these calls. Surpluscapacity is then allocated according to demand by the mobile terminals,subject to the maximum bit rate for each call.

While the above embodiment is described with reference to a satellitecommunications system, aspects of the present invention may also beapplied to terrestrial cellular communications systems. The mobileterminal may be portable, may be mounted on a vehicle or form part of atemporary or permanent installation, such as a temporary office buildingor a wireless telephone booth.

Elements of the embodiment are described in terms of functional blocks.These blocks do not necessarily correspond to discrete units, butfunctions of more than one functional block may be performed by onediscrete unit, or the function of one functional block may be performedby more than one discrete unit.

Although the preferred channel format is TDMA, aspects of the presentinvention may also be applied to CDMA communications systems.

What is claimed is:
 1. A method of registering a communications terminalwith a satellite communications system including a satellite generatinga plurality of spot beams each carrying at least one spot beamtransmission channel and a global beam substantially encompassing theplurality of spot beams and carrying at least one global beam receptionchannel and at least one global beam transmission channel, said methodcomprising: receiving a call on one said spot beam transmission channel;ceasing reception of said call; subsequently determining whether saidone spot beam transmission channel is receivable by said terminal; and,if said one spot beam transmission channel is not receivable,transmitting a registration message in said global beam receptionchannel; receiving a spot beam channel allocation signal in said globalbeam transmission channel; and receiving a further said spot beamtransmission channel selected according to said spot beam channelallocation signal.
 2. Apparatus for a satellite communications terminalfor use in a satellite communications system including a satellitegenerating a plurality of spot beams each carrying at least one spotbeam transmission channel and a global beam substantially encompassingthe plurality of spot beams and carrying at least one global beamreception channel and at least one global beam transmission channel,said apparatus comprising: a receiver for receiving a call on one ofsaid spot beam transmission channels and subsequently ceasing receptionof said call; means for subsequently determining whether said one spotbeam transmission channel is receivable by said terminal; and if saidone spot beam transmission channel is not receivable, transmitting aregistration message in said global beam reception channel; and meansfor receiving a spot beam channel allocation signal in said global beamtransmission channel and tuning said receiver to receive a further saidspot beam transmission channel selected according to said spot beamchannel allocation signal.